1. Field of the Invention
The present invention relates to the use of electron spectroscopy as a depth profiling or lateral differentiation probe useful, for example, for characterization of optoelectronic devices and other applications of mesoscopic systems, i.e. between the macroscopic solid and the atomic scale.
2. Prior Art
Progress in the development of new microelectronic technologies, leading to smaller, faster and smarter electronic and optoelectronic devices, depends on the ability to construct sophisticated interfacial structures which are well-resolved on the nanometer scale. One of the most acute problems in the development, study and application of such structures is finding characterization techniques suitable for such systems, namely having appropriate resolution in both the lateral and depth directions, the lateral direction being parallel to the surface of a sample at which such structure is disposed and the depth direction being perpendicular to the lateral direction. This is naturally of primary importance in the development stage, but also in the production stage, i.e., for on-line quality control. The continuing decrease in component dimensions, already approaching the nanometer range, poses formidable requirements on characterization methods.
X-ray photoelectron spectroscopy (XPS) is a powerful surface analytical tool, providing superior information on the chemical composition of surfaces and interfacial layers. The technique is based on illumination of the surface with X-rays and analysis of the photoelectrons ejected from the surface, thereby determining the identity and chemical state of atoms located on the surface and up to ca. 10 nm deep. In contrast to its nanometer-scale depth sensitivity, in the lateral direction XPS is essentially a macroscopic technique. Several XPS-based depth profiling methods exist, most prominently those based on ion etching or on analysis of line intensities at different detector angles (angle-resolved XPS). Both suffer from various drawbacks, including induced sample damage, distortions associated with non-planar surfaces, and others. As noted above, in the lateral direction, only macroscopic information is commonly obtained.
In addition, the quest to achieve well-defined features which are resolved on the nanometer length scale and distributed in predetermined patterns on solid surfaces is the heart of future optoelectronic devices and a major goal in science and technology. Structural analysis of such systems usually requires scanning probe methodologies, which are essentially small-area techniques. Large-area analytical tools, such as XPS, are limited in this respect. A fundamental feature of XPS is the contrast between its depth resolution and lateral resolution, typically approaching nm vs. xcexcm length scales, respectively. This raises serious problems in the study of non-planar or heterogeneous surfaces, it especially when surface variations fall in the region between the two length scales. For various applications, however, this intermediate region is the relevant one; hence, new high resolutionxe2x80x94large area characterization methods are crucially needed.
Electrical charging of the sample surface is commonly considered an obstacle to accurate experimental determination of binding energies in XPS measurements of poorly conducting surfaces6, 19, 20 To compensate the extra positive charge that is a natural consequence of photoelectron emission and stabilize the energy scale on a reasonably correct value, an electron flood gun is routinely used. Such gun creates a generally uniform potential across the studied volume. This, however, is often impossible with structures comprising components which differ in electrical conductivity9, 21-23 In such cases, chemical information may be smeared due to XPS line shifts that would follow local potential variations. On the other hand, this very effect can be used to gain structural information10-12. Several studies9, 24-26 have focused on various aspects of charging in XPS, indicating that, on a macroscopic scale, differential surface potential can be analyzed using a classical approach based on charge generation vs. discharge rates.8, 27 Application in surface analysis has been demonstrated.8, 10, 11,-27-29 
Thin layered structures a few nanometers in thickness impose demanding requirements on the depth sensitivity of analytical methods. X-ray photoelectron spectroscopy (XPS)1, an effective surface analytical tool providing superior chemical information, offers depth sensitivity inherently appropriate for such nanostructures. However, translation of the XPS integral line intensity into high resolution depth information is not straightforward. The commonly used XPS depth profiling methods, i.e., ion etching2, angular-resolved XPS (ARXPS)2, and Tougaard""s approach3, are effective but limited in various aspects2-5. Ion etching2 is inherently destructive and limited in application particularly with soft matter. ARXPS2, considered nondestructive, is hampered when applied to non-planar morphologies, it requires a large number of measurements (which may induce damage5), and is strongly model dependent4. Tougaard""s approach3 (quantitative analysis of signal-to-background correlation) requires minimal interference of neighboring lines across a wide spectral range, and is therefore less effective with small signals.
The present invention provides improvements in the observation of such structures by the use of selected controlled surface charging (CSC) in conjunction with electron spectroscopy. CSC is basically non-destructive, allowing fast and convenient data collection. It enables differentiation of spectrally identical atoms at different locations. It is applicable to thicker structures than ARXPS, as the signal is FE not subject to increased attenuation associated with off-normal measurements. The method according to the invention offers several advantages over existing depth profiling techniques and is of general applicability to a large variety of mesoscopic heterostructures. Substrate roughness has only a minor effect on CSC depth profiling. The linear dependence on depth, which occurs in systems that have been studied, is an attractive feature of CSC. However, the invention also offers advantages for the study of other systems that may present more elaborate conduction processes, possibly causing deviations from linearity. Such deviations may contribute to the exploration of additional characteristics of the systems, such as charge distribution, conduction mechanisms, etc.
One embodiment of the invention is a method of examining a sample, comprising: performing a first spectroscopic analysis of a surface portion of the sample when the sample surface portion is in a first electrical charge state; placing the sample surface portion in a second electrical charge state that is different from the first electrical charge state and performing a second spectroscopic analysis of the surface portion of the sample when the sample surface portion is in the second electrical charge state; and comparing the first spectroscopic analysis result with the second spectroscopic analysis result to obtain at least one of structural and electrical information about the sample, wherein the first and second electrical charge states are given values that enable information to be obtained about sample structural features having dimensions of less than 50 nm.
A second embodiment of the invention is a method of examining a sample, comprising: performing a first spectroscopic analysis of a surface portion of the sample when the sample surface portion is in a first electrical charge state; placing the sample surface portion in a second electrical charge state that is different from the first electrical charge state and performing a second spectroscopic analysis of the surface portion of the sample when the sample surface portion is in the second electrical charge state; and comparing the first spectroscopic analysis result with the second spectroscopic analysis result to obtain at least one of structural and electrical information about the sample, wherein each spectroscopic analysis result contains data identifying a characteristic of the spectral response for each of at least two elements contained in the sample, and the step of comparing includes: determining, for each of the at least two elements contained in the sample, a difference between the first spectroscopic analysis result and the second spectroscopic analysis result; and correlating the differences determined in the determining step for the at least two elements to provide a quantitative value of a characteristic of the sample. More specifically, the correlation is between the difference associated with one of the elements and that associated with the other of the elements.
One embodiment of apparatus for examining a sample according to the invention comprises: a spectroscopic analysis instrument including a component that places a surface portion of the sample in different electrical charge states, for performing a first spectroscopic analysis of a surface portion of the sample when the sample surface portion is in a first electrical charge state and performing a second spectroscopic analysis of the surface portion of the sample when the sample surface portion is in a second electrical charge state different from the first electrical charge state; and a comparator coupled to the instrument to compare the first spectroscopic analysis result with the second spectroscopic analysis result to obtain at least one of structural and electrical information about the sample, wherein the component is operative for giving the first and second electrical charge states values that enable information to be obtained about sample structural features having dimensions of less than 50 nm.
Another embodiment of apparatus for examining a sample according to the invention comprises: a spectroscopic analysis instrument including a component that places a surface portion of the sample in different electrical charge states, for performing a first spectroscopic analysis of a surface portion of the sample when the sample surface portion is in a first electrical charge state and performing a second spectroscopic analysis of the surface portion of the sample when the sample surface portion is in a second electrical charge state different from the first electrical charge state; and an arithmetic unit coupled to the instrument to compare the first spectroscopic analysis result with the second spectroscopic analysis result to obtain at least one of structural and electrical information about the sample, wherein each spectroscopic analysis result contains data identifying a characteristic of the spectral response for each of at least two elements contained in the sample, and the comparison performed by the arithmetic unit includes the steps of determining, for each of the at least two elements contained in the sample, a difference between the first spectroscopic analysis result and the second spectroscopic analysis result and correlating the differences determined in the determining step for the at least two elements to provide a quantitative value of a characteristic of the sample.
In one aspect, the invention provides a method for depth profiling of a sample consisting of a thin layer of dielectric material on a conducting substrate using controlled surface charging in electron spectroscopy. The term xe2x80x9cdepth profilingxe2x80x9d as used herein refers to the determination of the chemical composition of a sample in the direction perpendicular to the sample surface.
For depth profiling, the sample to be examined should be not more than 20 nm thick and preferably not more than 10 nm thick. The dielectric material may be, for example, without being limited to, ceramic materials (e.g. alumina), silica, metal oxides, polymers, organic multilayers and biological molecules. The conducting substrate may be of a metal, e.g. Au and Ag, or a semiconductor, e.g. Si, GaAs, or TiO2. The sample may be excited with any suitable irradiation source such as X-ray, electron beam, and UV, and the low energy source may be any suitable source such as electron beam or ion beam source. Procedures using an electron beam source will be described below. An ion beam source can be used in it the same way to produce different positive surface charge states. This will produce spectral peak shifts in the opposite direction from those produced by an electron beam source. The ion beam may be an AR+ source. The use of such a source to neutralize surface charge is described in Nelson et al, xe2x80x9cSurface charge neutralization of insulating samples in x-ray photoemission spectroscopyxe2x80x9d, J. Vac. Sci. Technol. A 16(6), Nov/Dec 1998, 3483-89.
According to one embodiment of the present invention, a novel application of XPS is presented, where accurate depth information is obtained from measured photoelectron energy values. Excess negative charge, stabilized on the surface of a dielectric overlayer by application of an electron flood gun, creates controllable potential gradients along the depth axis; the local potential is probed directly via XPS line shifts, providing the depth position of the atoms. This approach is described below with reference to self-assembled multilayers on gold surfaces, where marker monolayers are inserted at predetermined depths. Nanometer depth resolution, obtained on a linear energy scale, is shown, in excellent agreement with traditional line intensity analysis.
The examples presented hereinbelow use X-ray photoelectron spectroscopy (XPS). However, Auger electron spectroscopy (AES), scanning Auger, UPS, SEE and other spectroscopies may be appropriate. Local potential gradients are projected on the electron spectrum, thus providing unique structural and electrical information. The method is suitable for analyzing interfacial structures on the nanometer scale, including very thin films.
In another aspect, the invention makes use of known surface charging phenomena to analyze self-assembled (SA) monolayers on heterogeneous substrates, providing lateral resolution on a scale given by the substrate structural variations, i.e., much smaller than the probe size10. This aspect of the invention provides a simple approach where the superior depth resolution of XPS is used to gain lateral sensitivity to surface structural variations that meets the above requirements. It is based on controlled variation of a selected parameter, the excess surface charge, which is sensitive to differences in the local conductivity.
According to this aspect, XPS is used to analyze mesoscopic systems at a lateral resolution given by the substrate structure. The method is based on controlled differential charging of multi-component surfaces, using the electron flood gun or an analogous device.
With nm-size surface features, surface conductivity, adhesion quality, geometrical factors, etc., are complicating factors. Nevertheless, the resultant excess charge produced by the electron flood gun, and accordingly the XPS line shifts, provide a powerful tool for site classification in patterned surfaces. In contrast to the low level naturally occurring positive charging that occurs in mesoscopic structures, which is usually insufficient for satisfactory analysis, the use of the flood gun at relatively high fluxes, producing negative excess charge, leads to larger and well-controlled spectral line shifts, required for quantitative analysis and site classification.
Procedures according to this aspect of the invention can serve to obtain integral information, namely, differentiating between portions of the measured area. The emphasis here is on the correlation between a substrate and overlayers thereon. Hence, an important application would be the examination of a patterned substrate onto which a certain overlayer is applied. The method allows to separately analyze the overlayer on each of the different parts of the substrate.
As will become apparent from the following detailed description, methods according to the invention can serve a variety of purposes including, but not limited to, the following examples: determination of the breakdown voltages across thin dielectric layers; this is relevant to ultrathin capacitors, electronic devices, etc; quality control in integrated circuits; analysis of thin polymer films, with minimal induced damage; multilayer structures (design of new optoelectronic materials, biosensors, heterogeneous catalysts, etc.) including depth profiling and electrical tests; study of adsorption and layer formation on heterogeneous substrates; differentiation of species which are spectrally similar; analysis of lateral distribution of materials/elements on a surface (chip); and testing the electrical conduction and connectivity on microchips.
By proper selection of flood gun flux and bias voltage levels, the invention allows information, including qualitative information to be obtained for samples having lateral or depth features smaller than 50 nm, preferably smaller than 10 nm, and even smaller than 5 nm. Quantitative information can be obtained on the basis of relationships between two or more elements in the sample with respect to the binding energy level shifts in associated spectral response peaks resulting from changes in surface charge state.